Weather-stabilized asbestos roofing felt

ABSTRACT

THERE IS DISCLOSED AN ASBESTOS ROOFING FELT SHEET WITH IMPROVED DIMENSIONAL STABILITY UNDER EXPOSURE TO WHEATHER, AND HAVING AN INCREASED ASPHALT SATURANT-ASBESTOS FELT WEIGHT RATIO, WHEREBY NOT ONLY WEATHER RESISTANCE IS GREATLY IMPROVED, BUT ALSO THE SPEED OF MACHINE SATURATION OF THE FELT WITH HOT LIQUEFIED ASPHALT IS GREATLY INCREASED BY VIRTUE OF THE INCREASED POROSITY AND PERMEABILITY OF THE FELT. THE SHETS CONSISTS BASICALY OF A VERY OPEN-MESH SKELETAL STRUCTURE OF MINERAL WOOL FIBERS WHICH ARE CONSIDERABLY COARSER, LONGER AND MORE RIGID THAN ASBESTOS FIBERS, AND CONSTITUTE FROM ABOUT 5% TO ABOUT 1K% BY WEIGHT OF THE SHEET, WITH THE BALANCE OF THE FIBER CONTENT (FROM ABOUT 70% TO ABOUT 90% BY WEIGHT) BEING CHRYSOTILE ASBESTOS. A SMALL AMOUNT (FROM ABOUT 3% TO ABOUT 6% BY WEIGHT) OF A MODIFIED STARCH CONSTITUTES A BINDER AND INCREASES THE TENSILE STRENGTH OFTHE FELT, AND A SMALL AMOUNT (FROM ABOUT 4% TO ABOUT 8% BY WEIGHT) OF AN ORGANIC FIBER IS ALSO USED TO ACT AS A BINDER FOR THE FIBERS, AND FURTHER TO INCREASE THE FEXIBILITY OF THE FELTED SHEET. A FRACTIONAL PERCENTAGE OF A SURFACE ACTIVE AGENT MAY BE USED TO PROVIDE SMOOTHER MACHINE OPERATION AND TO YIELD A MORE UNIFORM FELTED SHEET.

April 24, 1973 mLDEBRANDT ET AL 3,729,373

WEATHER-STABILIZED ASBESTOS ROOFING FELT Filed May 6, 1971 INVENTOR/S 60/41. 52/510 4010542641?- 7/45000/95 1Q. M40651. 4155/87 r? #0264 505/5 ,4. .Sr'EK/t/ BY yw, 2;, WW

ATTORNEYS United States Patent O 3,729,373 WEATHER-STABILIZED ASBESTOS ROOFING FELT Guillermo J. Hildebrandt, Theodore R. Maugel, Albert R. Morgan, and Boris A. Stern, Cincinnati, Ohio, assignors to The Celotex Corporation, Tampa, Fla.

Filed May 6, 1971, Ser. No. 140,755

Int. Cl. D21f 11/04 US. Cl. 162-123 16 Claims ABSTRACT OF THE DISCLOSURE There is disclosed an asbestos roofing felt sheet with improved dimensional stability under exposure to weather, and having an increased asphalt saturant-asbestos felt weight ratio, whereby not only weather resistance is greatly improved, but also the speed of machine saturation of the felt with hot liquefied asphalt is greatly increased by virtue of the increased porosity and permeability of the felt. The sheet consists basically of a very open-mesh skeletal structure of mineral wool fibers which are con siderably coarser, longer and more rigid than asbestos fibers, and constitute from about 5% to about 15% by weight of the sheet, with the balance of the fiber content (from about 70% to about 90% by weight) being chrysotile asbestos. A small amount (from about 3% to about 6% by weight) of a modified starch constitutes a binder and increases the tensile strength of the felt, and a small amount (from about 4% to about 8% by weight) of an organic fiber is also used to act as a binder for the fibers, and further to increase the flexibility of the felted sheet. A fractional percentage of a surface active agent may be used to provide smoother machine operation and to yield a more uniform felted sheet.

BRIEF SUMMARY OF THE INVENTION This invention relates to an improved asbestos roofing felt sheet having greatly increased dimensional stability when exposed to the weather as a component of built-up roof coverings. Asbestos roofing felts, composed of asbestos fibers and binders, wet-laid to form a sheet on a paper machine, dried to remove the residual water and then saturated with liquefied hot asphalt, have been extensively used for many years in the construction of built-up roof coverings and generally have proved to be highly durable and weather resistant.

GENERAL CONCEPT OF THE INVENTION The general concept on which this invention is based is that an asbestos felt which is to be exposed to the weather for a long period of time as a component of a built-up roof covering needs two improvements that the present asphalt-saturated asbestos felts lack. First, the asbestos felt needs an integral, internal reinforcement of fibrous character, but which has greater resistance to dimension changes than the waterlaid asbestos sheet of which it is a part; and which also is more resistant to the effects of moisture than the asbestos fibers and organic fibers and binders in the sheet. Moisture changes in the asbestos felt due to wetting and drying during weather exposure have been found to be chiefly responsible for the poor weathering of some asbestos felt build-up roofings, accompanied by erosion of the surface and distortion or wrinkling of the asbestos sheet.

The second essential factor for improved weathering performance is an increased weight ratio of asphalt saturant to the weight of asbestos felt. The asphalt saturant is the only water-repellent component of the saturated sheet and an increase in the ratio of saturant to asbestos reduces greatly the access of moisture to the asbestos fibers and the organic binder material cementing the fibers in the sheet. This minimizes the effects of the wetting and drying that occurs in normal weather exposure and which may induce expansion, shrinkage, delamination and distortion of the felted sheet structure.

This invention provides the requisite improvements in a very simple and effective manner. The internal fibrous reinforcement of the asbestos sheet which it provides con sists of a very open-mesh, skeletal structure of inorganic (non-asbestos) fibers of considerably coarse diameter, greater length and greater rigidity than the fibers and ultimate fibrils of the asbestos. These fibers are of a special glass composition of the type of calcium aluminate silicate glass commercially available under the common name of a mineral wool. While it might be possible to produce an open-mesh fibrous mineral Wool web separately and then form the reinforced asbestos sheet by wet laying the asbestos fibers on the web during the formation of the asbestos felt on the paper machine, it is a great advantage of this invention that the open-mesh, glass fiber structural skeleton and the asbestos sheet which it reinforces are formed simultaneously from a wet machine slurry containing the asbestos fibers admixed with a minor proportion of the special glass fibers.

The kind of roofing felt with which this invention is concerned is produced from chrysotile asbestos fibers, of which ample supplies of suitable quality are available from mines in Canada and the U.S.A. The commercial grades of chrysotile fiber have been well standardized for many years under the Canadian Fiber Classification System of the Quebec Asbestos Manufacturers Association. Under this classification, Grades 5 and 6 are designated as paper grade fibers and are extensively used in the manufacture of asbestos papers and felts, although sometimes small proportions of Grade 4 or Grade 7 may also be included in the felt composition. About 6% by weight of starch is commonly used as a binder for the asbestos fibers to increase the tensile strength of the dry sheet. Sometimes a small amount of organic fiber, in the range of 2% to 8% by weight of the asbestos, is also included to improve the toughness and flexibility of the felt.

Asbestos roofing felt is produced as a waterlaid web on a special multicylinder paper machine. While a thin asbestos paper, up to about .010 inch thickness, can be produced on a single screen cylinder or a Fourdrinier type machine, roofing felts in the thickness range of .020 to .050 inch thickness are made on a machine having three to seven screen cylinders in line. The drainage of water from an asbestos slurry to form a waterlaid web is quite slow; this is generally believed to be due to the extreme fineness and flexibility of the asbestos fibers and their strongly hydrophilic surface character. The thin asbestos web formed on the first cylinder is transferred to a carrier felt and successive waterlaid webs are added progressively from the second, third and succeeding cylinders until the desired thickness has been built-up. The wet webs are integrated by pressure to form a single web, further de-watered by suction or pressure and then transferred to a bank of heated dryer rolls to remove the remaining water, leaving a final moisture content of not over 5% by weight. An asbestos roofing felt of .025 inch thickness may be composed of five integrated Wet web layers each of .005 inch thickness.

Thus, an asbestos roofing felt has a laminated structure and if the integration of the wet laid webs has not been properly done it may be subject to delamination during weather exposure. Also, due to the fineness of the fibers and their high density compared with organic fibers, the felt is inherently compact, of low porosity and with a low saturation capacity for asphalt. This is especially true with chrysotile asbestos of short length such as Q.A.M.A. Grade 6 and of chrysotile fiber which is classed as soft, as contrasted with chrysotile that is harsh or semi-harsh,

The harsh chrysotile fibers form a waterlaid web that drains faster on the paper machine cylinder and produce a more porous felt with greater asphalt saturant capacity than a similar felt produced from soft chrysotile. These differences and the problems connected with them have long been recognized in the asbestos paper industry, but until this invention no satisfactory solution had been found. Specifically, asbestos roofing felts made with soft chrysotile paper grade fibers tend to be too compact, have low saturant capacity and absorb hot liquefied saturant asphalt slowly in the machine saturator.

PURPOSES OF THE INVENTION The principal purpose of this invention is to provide an improved asbestos roofing felt which, when impregnated with a suitable roofing asphalt saturant, can be used in the construction of built-up roof coverings that possess greatly improved dimensional stability under weather exposure. The novel roofing felt of this invention is highly resistant to dimensional changes and to delamination so that the repetitive wetting and drying of the roofing which occur during weathering does not induce expansion or shrinkage and thereby cause distortion or Wrinkling of the asbestos felt, such as has caused unsatisfactory service or failure of some asbestos felt build-up roofings in the past.

Another purpose of the invention is to produce an asbestos roofing felt having an altered internal structure, embodying an integral, skeletal reinforcement of nonasbestos inorganic fibers, such that the porosity and the ease of impregnation of the felt with a hot liquefied asphaltic saturant are substantially increased. Ancillary to this purpose, the weight ratio of asphaltic saturant to the Weight of the asbestos felt is increased, to afford greater weather protection to the asbestos fibers and binders of which the felt is composed, also to improve the flexibility and toughness of the asphalt-saturated felt.

A further object of the invention is to produce an asbestos felt which by reason of its altered internal structure has substantially increased porosity (saturant capacity) and permeability both to air and to the hot, liquefied asphaltic saturant, whereby the machine speed of the saturation process in the plant can be increased for greater productivity.

An incidental object, of considerable economic importance, is to enable the use in asbestos roofing felts of certain shorter and less costly grades of asbestos fiber of the Canadian paper grade Standard Fiber Classification groups and of soft chrysotile asbestos, while still producing a felt that meets recognized specification requirements and performs satisfactorily as to Weathering in built-up roofing constructions.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic representation of a magnified cross-section of a waterlaid Web of mineral wool fibers.

FIG. 2 is a schematic representation of a magnified cross-section of a typical waterlaid web of asbestos felt as produced on a five-cylinder asbestos paper machine.

FIG. 3 is similar to FIG. 2 and shows schematically the internal structure of the improved asbestos felt of the present invention, as a magnified cross-section view.

The term Mineral Wool, as used herein to describe the nonasbestos fibrous material used in the improved asbestos felt compositions of this invention, refers to the well-known mineral fibers, of siliceous glassy composition, that are extensively produced and mainly used for the thermal insulation of homes, industrial buildings and equipment. Mineral wool is a true glass fiber, although of quite different composition than bottle glass, window glass, or so-called fiberglass, and contains little or no sodium oxide. The fibers are produced from a liquid, glass melt of siliceous, calcareous and argillaceous raw materials, which is attenuated or drawn into fibers by various methods, including blowing with air or steam jets and .4 spinning the fluid melt from a rapidly revolving disc or drum. The methods and equipment for producing mineral wool are well-known in the industry and need not be described in detail. The terms rock wool and slag wool are often used to indicate the source of the raw material used to produce the mineral wool, but do not otherwise differentiate the products.

Most of the commerically produced mineral Wool may be defined as primarily a calcium aluminum silicate glass, modified slightly by minor amounts of magnesium, manganese and iron oxides which are usually present in the composition. Mineral wool is produced from various readily available raw materials, including certain natural siliceous, calcareous and argillaceous rocks, iron blast furnace slag, phosphorus furnace slag, and some other metallurgical slags of siliceous character. A typical range of composition follows:

Composition, percent Major ingredients: by weight Silica (SiO 35-45 Alumina (A1 0 8-16 Calcium oxide (CaO) 25-45 Minor ingredients:

Magnesium oxide (MgO) 2-15 Manganese oxide (MnO 0.75-1.25 Iron oxide (Fe O 0.2-1.5 Sulphur oxide (50 Trace-0.8

Depending on the composition of the various raw materials and the balance between acidic and alkaline oxides, silica (acidic) may be added if necessary to render the glass chemically stable and resistant to the action of moisture. The mineral wool may contain substantially larger amounts of magnesium oxide and manganese oxide and still possess satisfactory properties for use in the improved asbestos felt, but it is preferable that the iron oxide content should not exceed about 10% because mineral wool with a higher iron oxide content tends to be finer and more flexible so that the fibers may have insufiicient rigidity to form the desired skeletal reinforcing structure according to this invention.

It is evident that While commercial mineral wool fibers are of special glass composition within the general ranges stated above and the properties of the fibers are influenced to some degree by differences in composition, the physical properties such as rigidity, fiber diameter and length, and Water resistance are the primary determinants of suitability for use in the weather-stabilized asbestos felt.

A mechanical reinforcement of continuous, flexible spun fiber glass threads, has sometimes been incorporated in asbestos paper or felt to increase its tensile strength and tear resistance and woven glass fiber fabrics have also been used in the same way. This use is based on a different concept from the present invention since the threads and fabric are prefabricated and then inserted into the asbestos felt as it is being formed on the paper machine, like inserting steel reinforcing rods into concrete. In any event, such a method would not achieve the results of the present invention because the inserted flexible glass threads are spaced apart and do not change in any Way the structure of the asbestos felt matrix of fibers and binder between the threads and thus fail to provide the requisite skeletal structure that stabilizes the felt against dimensional changes.

The principal use of mineral wool has been as a low cost thermal insulation material in various forms-as bulk (loose wool) or granulated wool for filling hollow wall spaces; as dry-felted rolls, batts or blanks for walls, ceilings and floors; and as wet-felted thick slabs containing a substantial quantity of binder, for roof insulation use. Another important commercial use is as a thermal insulating cement for covering pipes, boiler walls and other heated surfaces. Mineral wool insulating cement consists mainly of granulated Wool, with minor proportions of short asbestos fiber, diatomite or other finely divided heat resistant mineral fillers, and suflicient colloidal clay to impart plasticity and spreadability to the composition when mixed with water. The colloidal clay-plasticized mixture is troweled on the surface to be insulated and then dried in place to form a solid thermal insulating covering. Another use for mineral wool is in spray-on fire proofing compositions applied to the steel structural members of buildings to protect the steel framework from collapsing when exposed to interior fires.

It is of course well-known that mineral wool has been used in combination with asbestos fiber in certain products, for example, the mineral wool insulating cement cited above, but this does not negate the novelty of the composition and product improvement involved in the present invention, in which the mineral wool is a minor ingredient in amount but produces a major improvement in the properties of the asbestos felt. Reference may be made to U.S. Pat. 1,887,726 to Loius Weber, for one of the early attempts to produce a felt or heat insulating paper from mineral wool. The composition was 80% lead slag wool fiber, 15% asbestos fiber and colloidal clay, which is virtually the same composition as that of mineral wool insulating cement. This patented product had no commercial success because the mineral wool felt was too fragile and too difficult to manufacture; it did not have any of the properties required to enable it to be used as an inorganic roofing felt equivalent to the well-known asbestos roofing felts.

'It has of course long been recognized that asbestos and mineral wool fibers have certain common characteristics. Thus, both are relatively fine fibers of inorganic silicate composition that are highly heat resistant, completely inorganic, highly inert toward most chemicals, and moisture-resistant and weather-resistant. These resemblances have caused many attempts to substitute mineral wool for asbestos, but without success because the differences in properties between these two fibers are more significant than the resemblances in virtually all the products in which an inorganic fiber may be used in the composition. These fibers are therefore not equivalent for any of their industrial uses.

Among these important differences, mineral wool fibers are of non-hydrous, non-crystalline glass, predominantly of calcium aluminum silicate composition, whereas chrysotile asbestos is a hydrated, crystalline mineral of the formula: 3MgO.2SiO .2H O, from which the water of crystallization may be driven off by heating at temperatures above about 900 F. Mineral wool fibers are individual semi-rigid glass rods, varying in length and having diameters in the general range of 2 to 25 microns (thousandths of a millimeter). In commercial mineral wool the diameters are predominantly in the range of 2- 20 microns. Depending on their diameter the mineral wool fibers are more or less rigid, in any case of much greater stiffness or rigidity than the fibers of chrysotile asbestos. The diameter of the chrysotile asbestos fiber in the commercial grades varies, depending upon the degree to which the ultimate fibrils of the crystalline mass have been separated by mechanical crushing and screening in the asbestos milling process, but the ultimate fibrils have a diameter of about .02 to .04 mcron, roughly one-hundredth to one-fortieth of the diameter of the finest mineral wool fibers. Virtually none of the coarsest fibers of commercial chrysotile asbestos of papermaking grade are of diameter as large as that of the finest commercial mineral wool. Chrysotile asbestos fibers are inherently soft, very flexible and are not brittle or easily fractured.

Consequently, a waterlaid web or felt produced from mineral wool fibers has a radically different structure and properties than those of a felt produced from asbestos fibers. When mineral wool fibers are suspended in water to form a slurry and then waterlaid on a paper machine to produce a web, this web has what may be termed a brush heap structure, as illustrated schematically in FIG. 1. The web is extremely open and porous, semirigid, very low in tensile strength, and is extremely fragile and difiicult to handle without damage. By contrast, a slurry of paper grade crysotile asbesto fibers, prepared and waterlaid on a paper machine in the same manner to produce felt, makes an entirely diiferent structured sheet. The web is compact, tight and of low porosity, quite flexible and adequately strong for handling on the paper machine and dryers. 'Ihe cross-section structure of a typical asbestos felt is shown schematically in FIG. 2.

To accomplish the objectives of this invention and produce an asbestos roofing felt having improved dimensional and weathering stability, advantage has been taken of the differences above described by combining the two kinds of inorganic fibers into a structural entity which has properties much supreior to those of a felt made with either one alone.

DETAILED DESCRIPTION The improved, weather-stabilized asbestos roofing felt is produced from chrysotile asbestos of paper making grades, with 70-90% by weight of the composition being asbestos fiber, and preferably contains at least 75% of asbestos. The mineral wool fibers, of the general composition and physical characteristics described above are included as a strictly limited part of the composition from 5-15 by weight (preferably 6-13% by weight), not as a substitute for the asbestos but to produce a composite felt having improved properties not possessed by a similar felt made with asbestos fibers only. As a binder to cement the asbestos fibers together in the felted structure a small amount of a modified starch, preferably in the range of 36% by weight, is included in the furnish to increase the tensile strength of the felt. Oxidized cornstarch and acetylated starch are particularly suitable as binders in asbestos felt. A small amount of organic fiber, preferably in the range of 4-8% by weight, such as refined wood pulp is included, as this also acts as a binder for the asbestos fibers and increases the flexibility and handleability of the felted sheet. Chemically processed kraft fiber has been found particularly suitable for this purpose. The total amount of organic material (starch and fiber) in the composition is preferably limited to a maximum of 12% by weight.

An optional ingredient of the asbestos fiber-water slurry supplied to the paper machine is a surface active agent, added in a frictional percentage amount. Surface active agents improve the dispersion of the fibers in the slurry and help to provide smoother operation of the paper machine so that a more uniform felted sheet is produced. A suitable surfactant for this purpose is Ethomeen S-12 (brand name of an ethoxylated aliphatic amine produced by Armour & Co.) and it preferably is included in amounts from 0.1-0.5% by weight of the composition, but may be omitted if desired. As the surface active agent is Water soluble and if used is only a small fractional percentage of the composition, most of it is lost by filtration in the paper machine operation and only a trace remains in the finished felt.

It has been determined that the advantages of this invention are most effectively afforded by asbestos felt compositions within the following limited ranges as to each of the ingredients and as to total inorganic and ogranlc components.

Felt formula, percent by weight Total Asbestos Mineral inor- Organic Starch Total Preferred range fiber wool ganie fiber binder organic Minimum 75 6 88 4 3 7 Maximum 13 93 8 6 12 The preferred range of compositions for the improved weather-stabilized asbestos felt containing mineral wool fibers has been stated above. This range may be extended to somewhat broader limits for each of the ingredients and still achieve the important objective of producing a felt having greater dimensional stability than possessed by a similar felt made from the same grades of asbestos fiber, but containing no mineral wool. .Although asbestos felts produced from compositions near the limits of the broader range may not possess all the advantageous properties of those made within the preferred range they are considered to be within the scope of the broad concept of this invention.

Felt formula, percent by weight TYPICAL EXAMPLES OF THE INVENTION To illustrate the effectiveness of this invention in its several aspects-(a) improving the drainage of the wet laid asbestos web, (b) increasing the asphalt saturant capacity (porosity of the felt, (c) increasing the rate of absorption of the hot liquefied asphalt saturant by the felt to enable the saturation process to be carried out at increased speeds, while (d) retaining equal or better quality in respect to the other essential properties of asbestos roofing felt, two typical examples are presented.

The composition of each example is given below, compared with that of a conventional asbestos roofing felt having a composition typical of previous practice. All three felts were produced as hand sheets under uniformly controlled laboratory conditions and the comparative test results are given in Table II. The hand sheets were tested for the important dry felt properties and were then saturated with a suitable roofing asphalt (S.P'. 114 F.), by immersion for 1-0 seconds at 410 F. The finished asphalt saturated felts were tested for the essential properties of asphalt-asbestos roofing felt (weight, percent asphalt saturation, tensile strength, tear resistance, and pliability), with results as shown.

TABLE I.-ASBESTOS ROOFING FELT FORMULATIONS Percent by weight Conven- Typical tional example asbestos felt I II C hrysotile asbestos:

6R Grade 43.4 1 20.5 6D Grade.-. 43. 4 65. Total 86. 8 75. 5 85. 5 Mineral wool, granulated k.-. 12. 6 6. 0 Diatomite 2. 2 Refiberized organic roofing felt. 7. 6 Kraft ber 7. 4 5. 0 Oxidized cornstarch (Staley CTM) 3 4 4. 2 3. 3 Ethomeen surfactant 0. 3 0. 2

Total 100.0 100.0 100.0

1 Soft type.

2 The granulated mineral wool used was a calcium aluminum silicate glass and had the following composition: SiOr, 35.6%; A1203, 13.0%; MgO, 13.6%; CaO, 34.4%; MnO, F6203, 1.3%; 80a, 0.8%.

No'rE.The mineral wool fibers had diameters predominantly in the range of 2 to 20 microns.

The length of the mineral wool fibers of granulated wool varies generally in the range of to A, but in any case the average length of mineral wool fibers is several times as great as the average length of the paper grade asbestos fibers. In the preferred compositions of this invention, a substantial proportion of the mineral wool fibers are of a length greater than A" and at least 50 percent are greater than Ma".

TABLE IL-OOMPARISON OF PROPERTIES, DRY FELT AND ASPHALT-SATURATED Conven- Typical example tional asbestos felt I II Williams Freeness test on slurry, 500 00., sec./3 g. TAPPI modified RC-lll) method 313 43 106 Dry felt:

Weight, lb./1OO sq. ft 9. 5 9. 5 9. 5 Caliper, in. 025 028 026 Kerosene number (index of saturant capacity) 50. 7 66. 9 57. 1 Tensile strength, b1./1n. (average),

TAPPI T-404 28. 5 l9. 0 20. 0 Tear resistance, Elmendorf, g.

TAPPI T-414 153 162 172 Air porosity, densimeter, sec/ cc.

in. orifice) 232 58 128 Asphalt-saturated felt:

Weight, lb./100 sq. ft. 13. 7 14. 5 14. 0 Asphalt saturation,

Weight percent 43.8 53. 4 47. 2 Tensile strength, lb./in. (average),

TAPPI T-404 30. 2 25. 8 25. 9 Tear resistance, Elmendorf, g.

TAPPI T-414 286 419 402 Pliability, ASTM D-250, 77 F.,

radius mandrel 1 No cracks.

Referring to the data of Table II, it is evident from the Freeness Test results that the rates of dewatering and web formation from the slurry of both Examples I and II are much greater than for the conventional asbestos felt composition. Likewise, both examples of the dry felts show greatly increased porosity by densimeter test and have substantially increased asphalt saturant capacity (kerosene number).

Examination of the test results on the three asphaltsaturated asbestos felt finished products shows that Examples I and II are definitely superior to the conventional asbestos felt. Both have substantially higher asphalt saturant content--22% higher for Example I and 8% for Example II--and both examples have over 45% asphalt saturation. The tear resistance of both examples is higher than that of the conventional asbestos feltby 46% and 40.0%, respectively. The tensile strength values for the examples are slightly lower, but are entirely adequate for machine production, handling and application as built-up roofing. It should also be noted that the superior properties of Example II were obtained from an asbestos fiber composition of which a substantial proportion (24% of the total fiber) was of the soft chrysotile type.

TABLE III.-COMPARISON OF MACHINE-PRODUCED ASBESTOS ROOFING FELTS Conventional asbestos Typical felt Example I Dry felt:

Weight, lb.l100 sq. ft 9. 3 9. 6 Caliper, in 023 O25 Kerosene num ity 56. l 62 Tensile T404 21. 4 28 Cross tear resistance, Elmendorf, g. TAPPI T-414 133 Air porosity, densimeter, sec./100 cc. (1 in.

ori ce) 270 40 Asphalt-saturated felt:

Weight, lb./l00 sq. ft c. 13. 4 14. 2 Asphalt saturation, weight percent ASTM D-228 42. 1 49. 7 Tensile strength, lb./in., M.D. TAPPI T-404 33.1 35. 4 Cross tear resistance, Elmendorf, g. TAPPI Weather exposure test, 30 day 1 No cracks (cracks on mandrel). 2 No cracks (cracks on mandrel). 3 Slight distortion.

4 No distortion.

To confirm that the laboratory hand sheet results in Table II correspond to the properties of asbestos roofing felts as actually produced on factory equipment, the test results in Table III are presented. The novel asbestos felt composition of Example I was produced on a commercial asbestos paper machine and its properties compared with those of a conventional asbestos roofing felt produced on the same machine. Both felts were saturated with a standard roofing asphalt on the plant saturating equip ment and the properties of the two saturated felts were compared.

The test results on the dry felt produced from the novel composition of Example I show that it has greater tensile strength, greater tear resistance, greater asphalt saturant capacity (kerosene number), and much higher porosity (densimeter value) than the conventional asbestos felt.

In converting the two dry felts to asphalt saturated felts it was found that the greater porosity and higher saturant capacity of the felt of Example I made it possible to saturate the felt at much higher machine speed and at the same time increase the percent of asphalt saturant absorbed by the felt. Thus, the conventional asbestos felt could be properly saturated at an average speed of 160 f.p.m., whereas the novel felt of Example I could be properly saturated at speeds between 250 and 300 f.p.m. The felt of Example I contains 18% more saturant asphalt in percent of its dry felt weight than does the conventional asbestos felt. Also, the cross tear resistance of Example I is about greater than that of the conventional asbestos felt. Both saturated felts meet the minimum pliability requirements of ASTM D- 250, when tested on a /2 inch radius mandrel, but Example I is actually more pliable as it does not shown any evidence of cracking on the /4 inch radius mandrel but the conventional felt does crack on that size mandrel.

The weather exposure tests on the two saturated asbestos felts show that the major objective of this invention, which is to produce an asbestos roofing felt possessing superior weather stability, has been achieved in Example I. This felt shows no distortion in the 30 day weather exposure test, where the conventional asbestos felt does show slight distortion and is rated only fair as to weather stability.

Preparation of mineral wool for use in asbestos felt I furnish Due to the loose bulky form in which mineral wool is produced by the blowing or spinning process it is not very convenient for mixing and dispersing the fibers in an asbestos fiber-water slurry for the paper machine although this kind of mineral wool can be used if desired. Also, mineral wool as produced always contains a small proportion of non-fibrous material, generally in the form of small spherical or irregular particles of the glass composition that were not completely converted to fibrous form, termed shot. As this shotty material is of no benefit to the asbestos felt structure it should be removed which is conveniently done by the wellknown granulating process. The mineral wool industry processes a considerable part of its production by subjecting the loose fiber to a shredding process followed by forming the fibers into small pellets or granules which are convenient for pouring into hollow wall spaces or for mixing with other materials to produce insulating cements, etc. Also, the screening associated with the granulation process removes most of the non-fibrous shot. Such processed granulated mineral wool has been found particularly suitable for use in the asbestos felt formulations of the present invention. The pellets or granules of granulated mineral wOOl are generally in the range of A" to size, with at least 50% by weight retained on a A" standard test sieve.

In preparing the asbestos slurry for use on the paper machine the usual procedure is followed of mixing and dispersing the asbestos and organic fibers in the proper proportion of water, followed by addition of the starch and if desired the surface active agent. The mineral wool is added last to ensure that the fibers are subjected only to sufficient mixing to ensure their uniform and thorough dispersion among the asbestos fibers in the slurry and to minimize excessive mechanical agitation or abrasive action on the fibers that would tend to fracture them to shorter lengths and detract from their effectiveness in forming the requisite skeletal reinforcing structure in the wet laid asbestos sheet.

Referring to the drawings,

FIG. 1 shows schematically in enlarged cross-section the characteristic open, brush heap structure formed by a slurry of mineral wool fibers in water when deposited on a paper machine screen to form a waterlaid web. This Web has some flexibility, but very low tensile strength and is quite fragile and difficult to handle without mechanical damage. The semi-rigid, glassy mineral wool fibers do not intertwine and mat together to form a compact, strong web as paper grade asbestos fibers do.

FIG. 2 illustrates schematically the cross-section structure of an asbestos felt formed with five wet laid webs on a five-cylinder paper machine from a typical furnish of paper grade asbestos fibers in water. The wet webs are integrated by pressure in the paper machine process, but it is evident that the structure is laminated and that the laminations are actually held together by the intertwining of the asbestos fibers at the interfaces, plus the binder included in the formula. If the water content is too low or the pressure insufficient at the time the webs are integrated, defective bonding of the web layers may result so that the finished felt may be subject to delamination when subsequently exposed to the stresses involved in weathermg.

FIG. 3 is a schematic representation of an enlarged cross-section of the improved asbestos felt of the present invention, as produced on the paper machine for five waterlaid webs integrated into a unitary sheet, using an asbestos fiber furnish similar to that shown in FIG. 2, but including a limited proportion (5 to 15 by weight) of semi-rigid, glassy mineral wool fibers in the slurry. The difference in structurue is clearly evident and it can be seen how the semi-rigid mineral wool fibers form an openmesh structure within each of the webs to stabilize them against dimensional changes and to interlock the webs at their interfaces and thereby minimize any tendency to delamination of the dried integrated sheet. Since the mineral wool fibers are such a small proportion of the total fibrous material in the sheet, the felt retains the essential characteristics of an asbestos felt, but possesses greatly improved dimensional stability, has substantially increased asphalt saturant capacity and saturates much more rapidly than a conventional asbestos felt.

Exposure test for weather stability of asbestos roofing felt In asbestos built-up roof coverings, multiple layers of the asphalt-saturated asbestos felt are coated and cemented together with hot asphalt or a cold asphalt cement and the weather exposed surface of the uppermost layer is also coated. Thus, the asphalt saturated felt is not directly exposed to the weather and it may be a long time before distortion due to lack of dimensional stability becomes evident. However, in the actual construction of built-up roofings there is often a delay in the application of the coating of hot asphalt to the top, exposed surface and during this period of exposure wetting by rainfall and drying from the suns heat may induce distortion of the felt. Also, some asbestos built-up roofs are given a top surface coating of asphalt emulsion (which consists of asphalt dispersed in water). This of course involves the direct contact of a substantial amount of water with the felt surface until the water content of the emulsion has evaporated.

An exposure test has been devised which enables one to determine this important characteristic of the felt in much 1 1 less time and to compare the behavior of experimental felts of unknown stability with that of felts which have a record of satisfactory performance as to stability in actual service in built-up roofings.

This test consists of exposing the asphalt-saturated asbestos felt directly to the weather as the uppermost layer of a two-ply built-up roofing, but without any coating of weather protective asphalt on its top surface. The base layer of asbestos felt is cemented to the top, exposed layer with hot asphalt in the usual manner and the test panel is exposed to the weather in a horizontal position or with a low slope or pitch. Under such exposure conditions an asbestos felt that is deficient in dimensional stability may show incipient distortion Within a few days, although in dry weather this may take somewhat longer. For experience with this test method, an exposure of 30 days is considered adequate to differentiate between stable and unstable felts, but the exposure may of course be continued for an extended period to confirm the conclusions or to observe differences in degree of distortion that may occur. Generally, if no distortion, expansion, shrinkage, or wrinkling of the felt surface has appeared after 30 days of exposure the asbestos felt is regarded as highly stable and resistant to such defects.

Organic fibers and starch as binders in asbestos roofing felt The properties of asbestos roofing felt are improved by including a small amount of organic fiber in the composition to increase its tensile strength, toughness, flexibility and resistance to cracking. Fibers such as sulfite pulp, refiberized organic roofing felt and newsprint may be used, but the best results are obtained with a chemically processed wood fiber such as kraft pulp.

With asbestos fibers of paper-making grade a binder is required to provide adequate tensile strength for machine production, asphalt saturation and handling, and the roof application of the felt. Starch has been found most suitable as only a small amount is needed. Various kinds of starch, including tapioca, corn and potato starch have been used, but a modified cornstarch such as chlorinated (oxidized) or acetylated cornstarch is preferred due to its uniformity of viscosity, ease of preparation, and high binding strength for the fibers. An oxidized cornstarch sold under the brand name of Staley CTM (A. E. Staley Mfg. Co.) has been found entirely suitable and equivalent oxidized starches are available from other producers.

The invention has now been fully disclosed both as to its basic concept and as to the composition and methods for producing an asbestos roofing felt having very high weather stabiilty. Within its scope many variations are possible and no limitations are intended except those expressed in the following claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An asbestos roofing felt sheet having improved d1- mensional stability under weather exposure, consisting predominantly of chrysotile asbestos fibers of paper making grade, in the range of about 70% to 90% by weight of the composition, said felt being an integrated sheet of multiple waterlaid Webs, said felt composition containing a minor proportion of mineral wool fibers of larger diameter, and greater length and greater rigidity than said asbestos fibers, in the range of about to about 15% by weight, said mineral wool fibers having diameters predominantly in the range of about 2 to about 25 microns, said asbestos felt containing from about 3% to about by weight of organic fiber and from about 2% to about 7% by weight of starch, said organic fiber and said starch being binders for the inorganic fibers in said asbestos felt, said mineral wool fibers constituting an open-mesh, skeletal reinforcing structure within said waterlaid asbestos sheet, whereby said sheet is rendered highly resistant to expansion, shrinkage, distortion and delamination induced by the repetitive wetting and drying of the felt that occurs 12 during its exposure to the weather as a component of built-up roof coverings.

2. An asbestos roofing felt sheet, according to claim 1 in which said mineral wool fibers are predominantly of calcium aluminum silicate composition containing not more than about 10% by weight of iron oxide.

3. An asbestos roofing felt sheet according to claim 1 in which said chrysotile asbestos fibers include a substantial proportion of the soft chrysotile type of asbestos.

4. An asbestos roofing felt sheet according to claim 1, in which said felt is a unitary sheet consisting of from 3 to 7 integrated waterlaid webs, each of said webs having an open-mesh, skeletal internal stabilizing structure of mineral wool fibers.

5. An asbestos roofing felt sheet according to claim 1, in which said starch binder is a modified starch selected from the class of oxidized cornstarch and acetylated cornstarch.

6. An asbestos roofing felt sheet according to claim 1, in which said organic fiber is a chemically processed kraft wood pulp.

7. An asbestos roofing felt sheet according to claim 1 which is impregnated with roofing asphalt in amount at least 45% by weight of the dry felt.

8. An asbestos roofing felt sheet having high dimensional stability under weather exposure, consisting predominantly of chrysotile asbestos fibers of paper making grade, in the range of about 75% to about 87% by weight of the composition, said felt being an integrated sheet of multiple waterlaid webs, said felt composition containing a minor proportion of mineral wool fibers of larger diameter, greater length and greater rigidity than said asbestos fibers, in the range of about 6% to about 13% by weight, said mineral wool fibers being predominantly in the range of about 2 to about 20 microns diameter, and of lengths predominantly greater than inch, said asbestos felt containing from about 4% to about 8% by weight of chemically processed kraft wood pulp as an organic fiber and from about 3% to about 6% by Weight of oxidized cornstarch as a binder for said fibers, said mineral wool fibers constituting an open-mesh, skeletal reinforcing structure within said waterlaid asbestos sheet, whereby said sheet is rendered highly resistant to expansion, shrinkage, distortion, and delamination induced by the repetitive wetting and drying of the felt that occurs during its exposure to the weather as a component of built-up roof coverings.

9. An asbestos roof felt sheet according to claim 8, which is impregnated with roofing asphalt in amount at least 45% by weight of the dry felt.

10. An asbestos roofing felt sheet having high dimensional stability under weather exposure, consisting predominantly of crysotile asbestos fibers of paper making grade, in the amount of about 75.5% by weight of the composition, said felt being an integrated sheet of multiple waterlaid webs, said felt composition containing a minor proportion of mineral wool fibers of larger diameter, greater length and greater rigidity than said asbestos fibers, in the amount of about 12.6% by weight, said mineral wool fibers being predominantly in the range of about 2 to about 20 microns diameter and of lengths predominantly greater than A; inch, said asbestos felt containing about 7.4% by weight of chemically processed kraft wood pulp as an organic fiber and about 4.2% by weight of oxidized cornstarch as a binder for said fibers, said mineral wool fibers constituting an open-mesh, skeletal reinforcing structure within said waterlaid asbestos sheet, whereby said sheet is rendered highly resistant to expansion, shrinkage, distortion, and delamination induced by the repetitive wetting and drying of the felt that occurs during its exposure to the weather as a component of built-up roof coverings.

11. A11 asbestos roofing felt sheet according to claim 10, which is impregnated with roofing asphalt in amount at least 45% by weight of the dry felt.

12. An asbestos roofing felt sheet having high dimensional stability under weather exposure, consisting predominantly of chrysotile asbestos fibers of paper making grade, in the amount of about 85.5% by weight of the composition, said felt being an integrated sheet of multiple waterlaid webs, said felt composition containing a minor proportion of mineral wool fibers of larger diameter, greater length and greater rigidity than said asbestos fibers, in the amount of about 6.0% by weight, said mineral wool fibers being predominantly in the range of about 2 to about 20 microns diameter and of lengths predominantly greater than 4; inch, said asbestos felt containing about 5.0% by weight of chemically processed kraft wood pulp as an organic fiber and about 3.3% by weight of oxidized cornstarch as a binder for said fibers, said mineral wool fibers constituting an open-mesh, skeletal reinforcing structure within said waterlaid asbestos sheet whereby said sheet is rendered highly resistant to expansion, shrinkage, distortion, and delamination induced by the repetitive wetting and drying of the felt that occurs during its exposure to the weather as a component of built-up roof coverings.

13. An asbestos roofing felt sheet according to claim 12, wherein the chrysotile asbestos component is composed of 65.0% of 6D Grade and 20.5% of SR Grade and said R asbestos is of the soft chrysotile type.

14. An asbestos roofing felt sheet according to claim 12, which is impregnated with roofing asphalt in amount at least 45% by weight of the dry felt.

15. An asbestos roofing felt sheet having improved dimensional stability under weather exposure, consisting of at least 70% by weight of chrysotile asbestos fibers of paper making grade, said felt being an integrated sheet of multiple waterlaid webs, said felt composition containing a minor proportion of mineral wool fibers of larger diameter, greater length and greater rigidity than said asbestos fibers, in the range of 5% to 15% by weight, said mineral wool fi'bers having diameters predominantly in the range of 2 to 25 microns and of lengths predominantly greater than A inch, said asbestos felt containing from 5% to 15 combined total weight of organic fiber and starch as binders for said asbestos and mineral wool fibers, said mineral wool fibers constituting an open-mesh, skeletal reinforcing structure within said waterlaid asbestos sheet, whereby said sheet is rendered resistant to expansion, shrinkage, distortion and delamination induced by the repetitive wetting and drying of the felt that occurs during its exposure to the weather as a component of built-up roof coverings.

16. An asbestos roofing felt sheet having improved dimensional stability under weather exposure, consisting of at least by weight of chrysotile asbestos fibers, of paper making grade, said felt composition containing a minor proportion of mineral wool fibers of larger diameter and of greater average length than said asbestos fibers, in the range of 5% to 15 by weight of the composition, said mineral wool fibers having diameters predominantly in the range of 2 to 25 microns, said asbestos felt containing at least about 5% total weight of organic fiber and starch as binders for said asbestos and said mineral wool fibers, said mineral wool fibers constituting an open-mesh, skeletal reinforcing structure within said waterlaid asbestos sheet, whereby said sheet is rendered resitsant to expansion, shrinkage, distortion and delamination induced by the repetitive wetting and drying of the felt that occurs during its exposure to the weather as a component of built-up roof coverings.

References Cited UNITED STATES PATENTS 2,555,401 6/1951 Fasold et a1. 162-155 2,225,100 12/ 1940 Clapp 162-145 S. LEON BASHORE, Primary Examiner P. CHIN, Assistant Examiner US. Cl. X.R. 

